The use of a piston style cylinder is very common in semiconductor processing stations. In the piston style cylinder, a wafer carrier is supported by a piston, and the piston is set to move back and forth between a first position and a second position inside the cylinder. General speaking, wafers are very fragile, so they are expected to move at a constant speed to avoid damage resulting from abrupt acceleration or deceleration of a supporter such as a wafer carrier or a piston. Even though the supporter is initially set to move constantly, as time passes by, sludge might accumulate on the interior walls or at corners of the cylinder, and this would hinder the piston from constant reciprocal movement.
For transferring wafers, air is fed through a valve to a lower section of the cylinder while the piston is moving upward from the first position to the second position, and then exhausted while the piston is moving downward from the second position to the first position.
Meanwhile, air is fed through another valve to an upper section of the cylinder while the piston is moving downward from the second position to the first position, and exhausted while the piston is moving upward from the first position to the second position.
In the prior art, control circuitry with high precision sensors such as linear variable differential transformers (LVDT), voltage meters, or encoders is required for monitoring the status of the piston while the piston is moving back and forth between the first position and the second position. Such sensors are generally cost a lot. Therefore, there is a need to find a less expensive approach for monitoring the movement behavior of the piston in the cylinder.